The PON system is a high speed/large capacity optical access system in which an optical fiber transmission line and/or the OLT are shared by the plurality of ONUs, and attracts attention as a technique capable of suppressing a cost per user. In this PON system, an optical signal (downlink signal) from the OLT to the ONU is a continuous signal. On the other hand, an optical signal (uplink signal) from the ONU to the OLT is subjected to transmission control in the OLT in order to prevent signal collision to be a burst signal, and besides, a distance between the ONU and the OLT is different from the other depending on a user, and therefore, the OLT receives optical signals having different levels. Therefore, the OLT is required to have a receiver capable of achieving high reception sensitivity and a wide dynamic range.
As a method of achieving the wide dynamic range, a method with using a transimpedance amplifier for switching a gain in accordance with an input level of the burst signal is widely used. Patent Document 1 discloses a transimpedance amplifier which compares an instantaneous value of an output voltage with a threshold voltage and controls a gain switching circuit in accordance with its result. This method shows a configuration in which, by providing hysteresis characteristics to the gain switching determination circuit, a gain switching operation is performed based on a result compared and determined by a first hysteresis characteristic, and then, the gain switching operation is stopped based on a result compared and determined by a second hysteresis characteristic having a lower voltage detection level than that of the first hysteresis characteristic, so that the gain is fixed.